Construction of Earth Dam

7/27/2019 Construction of Earth Dam

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Construction of Earth Dam

P. Ghosh



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Phases of Construction

• Evaluation of plans, specifications, basic

requirements, and features of the site

• Planning and scheduling of the job

• Making the site ready

• Building the structure

• Clean up



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• The first phase of evaluation consists of a detailedstudy of the requirements of design and of the site

itself, including topography, weather and river flow

and of the problems of labour, materials, and

equipment supply

• The second phase is most important if the job is to

be done economically. The plant, labour, and

materials for each stage of construction process

must be available at the correct time; if they are too

early there is unnecessary down time, while if toolate the remaining schedule suffers


Contd..



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• Site readying includes developing access roads and

the provision of the fixed construction plants such

as ware houses, crushers, concrete mixers, offices

and housing for the staff and workers

• The major part of the time and money is spent on

the building phase

Contd..



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Site Preparartion

Clearing:

• It is the removal of tress, boulders, fences,

buildings, and other structures from the area. This

is for the borrowpits, and the area to be occupied by

the dam

Grubbing:

• It is the removal of roots, stumps and organic

matter. Scattered and isolated roots can be left, but

stumps and matted roots should be removed.

Grubbing is done only in the actual borrow pits and

within the outline of embankments



Stripping:

• It is the removal of all topsoil, organic materials and

soft pockets which are unsuitable for the borrow inthe borrow pit areas or unsuitable for supporting

structure within the embankment outlines

Scaling:

• It is the removal of loose materials, weathered rock

and weak materials from the abutments. It also

includes the shaping of abutments to eliminate

overhangs and provide uniform slopes 6

Contd..

Site Preparation



Diversion:

• Diversion is the re-routing of the river around the

construction area. This is done with a temporary dam or

cofferdam which walls off and protects the area, and a

diversion channel or tunnel which carries the watearound the site

Cofferdam fills:

• Many types of cofferdams are used depending on the

materials available and the river currents which flow

past it 7

Diversion and Cofferdamming

Di i d C ff d i




• The simplest is a fill of rock and earth dumped in

place to form a crude dam, Fig. 1

• The heart of this is a triangular mound of the

coarsest material obtainable such as boulders and

large broken rock

8

Contd..

Fig. 1



• This is dumped, starting from the abutments on both

sides and working toward the centre of the cofferdam

• If it is constructed in a flowing stream, the rocks

placed on the bottom must be coarse enough to resist

being carried away by the water velocity

• As the centre of the stream is approached, larger rock

must be used since the velocity increases as the flow

is restricted

• The final closure is effected with very large rocks o

even pre-cast blocks of concrete which are quickly

dumped in the gap9

Contd..


Di i d C ff d i



Combined coffer-dam:

• In some dams it is possible to utilize a rock toe of the

permanent embankment as the rock core of the coffer-

dam and thus reduce the overall cost of the cofferdam

(Fig. 2)

• Alternatively the rock toe can be made wider than

normal to compensate for the poor foundation andplaced directly on the un stripped ground

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Contd..


Fig. 2

Di i d C ff d i



Combined coffer-dam:

• In dams with an upstream core it is frequently possibleto use the upstream part of the dam itself as a second

stage cofferdam to protect the remaining foundation

area• A small cofferdam, quickly built after the flood season

is over, permits construction of the upstream part of

the dam

• This becomes the cofferdam for the next flood season

(Fig. 3)

11

Contd..


Fig. 3




• When the currents are swift and no coarse rock orboulders are available, cellular cofferdams of stee

sheet piling are used, Fig. 4

• Several patterns of cells are used depending on the

height. The circular cells are the most stable since

each cell is structurally independent of its neighbor

Contd..

Fig. 4



Cut-off trench:

• The soil-filled trench is most widely used form of cut-

off for earth dams on an earth foundation. It is also

used for the earth dams on rock where the upper

surface of the rock is weathered or cracked

• The construction procedure is employed is governed

by the nature of the foundation material, and in

particular by the ground water and the stable

excavation slope 13

Foundation Preparation

F d ti P ti



• For greatest economy an open excavation is used.

This is dewatered by pumping from sumps when the

foundation is made up of rock, clayey soils, or gravels

• The sumps, which are merely collecting pits, shouldbe dug just outside the trench line so that their

pumping will not interfere with the surface

preparation for the trench

• In silty soils and sands, sump pumping often creates

an unstable condition including seepage erosion, sandboils, and quick sand. To avoid this water table must

be lowered ahead of the excavation work so that

there is never an upward flow of water in to the

excavation14

Contd..




• This is best done by well points, Fig. 5, if the water table is to be lowered by 50ft or less

15

Contd..


Fig. 5

F d ti P ti



Impervious contact:

• The preparation of the contact between the cut-off (or

core when no cut-off is used) and the impervious

foundation requires careful hand work

• All loose materials and porous materials are removed.

Pits and crevices are cleaned out as deep as possible

• Thin horns and over hanging projections are removed

because they will interface with compaction and may

break off under the action of heavy equipment 16

Contd..


F d ti P ti



• If foundation is rock, it can be cleaned cheaply by a

combination of water-jetting and compressed air

• The small pits and cracks in rock must be filled with an

impervious material that resists washing so that a zone

of seepage will not develop along the contact

• Cement-sand mortar is best because it penetrates the

cracks easily and then sets up as a solid

• A clay paste can be used for the same purpose in the

smallest cracks 17

Contd..




Shell foundation:

• The foundation preparation beneath the shell is less

critical. Seepage is not a factor and no crack sealing

or pit filling is required

• All soft zones are removed and the embankment

placed in layers on the virgin surface

• Scarifying and rolling the foundation is sometimes

advocated, but it is an unnecessary expense whencontact is not a problem

18

Contd..


F d ti P ti



• If the foundation is of a loose, cohesionless soil with

a relative density of less than 70%, densification is

desirable

• Certainly a blanket of dense material 25 to 30 ft

thick is desirable to minimize the effects of any

structural readjustments in a loose foundation

• Three methods have been used to densify existing

deposits of cohesion less soil: a) pile driving,

b) explosives, and c) vibrofloatation19

Contd..


G ti



Grouting

• Grout is a fluid mixture of Portland cement, water and

various admixtures such as sand, clay and rock dust,

which hardens to form a semi-solid or solid mass that

is impervious and which has some degree of rigidity

and strength

• Clay grout is a viscous fluid mixture of clay and water,

plus sand and other filters, which tends to stiffen

slightly by the squeezing of water from the mix 20

G ti



• Chemical grouts are fluid mixtures of various

chemicals which solidify by chemical reaction after

their injection

• The process of injecting any of these materials in to

the ground is grouting

• Grouting is a complex process, an art in which the

experience and judgment of the operator is the most

important guide21

Contd..

Grouting

Grouting



Objectives:

a) Make the formation impervious

b) Improve its structural capabilities (strength,

compressibility)

• This is done by filling cracks, fissures, cavities and

even the voids between the grains with the grouting

material

• In most earth or rock fill dams the stress

concentrations are not sufficiently severe to require

structural improvement; in masonry dams, however this is a major factor

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Contd..

Grouting

G ti



Consolidation grouting:

• Consolidation grouting is the injection of a larger area

of the foundation to shallow depth to reduce its

compressibility, increase its strength, and particularly

to minimize seepage in slightly fractured or

weathered zone which is so often present in the

uppermost few feet of rock

• In earth dams it is limited to the contact zone with

the core or cut-off or to a limited portion of the

contact if it is wide23

Contd..

Grouting

Grouting



Curtain grouting:

• It is the establishment of a cut-off wall by grouting

• The depth of the curtain is determined from a

consideration of the seepage characteristics of the

foundation

• When the fissure pattern is so erratic that no rationa

analysis is possible the depth in rock is oftenestablished by an empirical procedure such as:

Depth = 0.5∆h to 1.5∆h

or 0.33∆h + c

where, ∆h is the head on the foundation, c is a constant

equal to 25 ft in sound rock and 75 ft in fissured rock24

Contd..

Grouting

Borrow Pit Excavation



• Excavation of the embankment materials, processing

them, and hauling them to the job are important from

considerations of both cost and the quality of the

materials delivered into the embankment

Excavation method:

• The selection of excavation procedure depends on

the type of available materials and on the

configuration of borrow pits

• Five principal methods for handling the materials

hand, power shovel, dragline, scraper and elevatinggrader

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Hand excavation

• It has been used throughout the history and is still

important in areas where labour is cheap

• Most types of soils can be excavated; but coarse

gravels, boulders, and even soft rock cannot be

handled so efficiently

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Contd..





Power shovel

• It is well adapted to a wide variety of materials from

soft soil to boulders and layered rock

• It is also suitable for hard rock that has been

shattered by explosives

• Stratified materials are easily mixed since the shovel

makes a vertical cut or slice





Drag line

• It is adapted to most soils except soft sticky clays and

very hard or cemented materials

• Stratified materials can be mixed or they can be

segregated, depending on the mode of operation





Scraper

• It is most efficient in slightly clayey sands, silts, non-sticky clays and it can even function in some soft

rocks

• Hard materials such as cemented soils and layeredrock can frequently be broken up with a rooter (a giant

hooked plough drawn by a tractor) and then excavated

with the scraper

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Contd..





Elevated grader

• The working of an elevated grader is similar to that of a scraper so far as the handling of various types of

materials is concerned

• It has, however, some tendency to provide more

pulverizing than the scraper

• A long level borrow pit is required for best operation

Contd..




Moisture control:

• Moisture control is necessary in the borrow pit to

permit efficient operation of the excavation

equipment and to condition the soil for its future

compaction

• Drainage is necessary for pits below ground water.

This should be done far enough in advance of the

construction so that the soil is not water logged

• Otherwise the soil must be stockpiled on higher

ground to drain and then re-excavated for use in the

embankment31

Contd..




• All pits should be so excavated that the surface

water drains away from them and does not collect in

them

Material control:

• The operation of the borrow pit must be carefully

controlled for better quality

• A testing laboratory is set up at the borrow pit where

the moisture content, gradation, and even plasticity

can be determined in minimum time32

Contd..




Modification:

• In some cases modification or processing is necessary

before the materials can be used

• The materials necessary for filters rarely occur in the

correct gradation

• Even though natural sand-gravel mixtures are found

which approximate the requirements, they are variable

and should be screened to provide more exact control33

Contd..

Fill Placement



• The placement of materials in the fill depends on

the method of hauling, the processing which will be

done in-place before compaction, and the size of the

area to be filled

• The materials hauled by hand are dumped in small

piles to form a reasonably level surface

• The materials hauled by trucks and wagons are

dumped in uniformly spaced piles

• Certain side dump wagons can also form long

narrow piles or wind-rows34

Fill Placement



In-place processing:

• All objectionable materials are removed during and

immediately after the spreading operation

• These include roots, clumps of grass and the other

organic matter, and large stones which would

interfere with compaction

• In some cases it is necessary to mix two materials.

An examples in core construction when a limited

amount of highly impervious clay can reduce the

permeability of more abundant but less water-tight

soils35

Contd..

Fill Placement



• If the materials are hauled by trucks or wagons they

are placed in alternate piles or in parallel adjacent

wind-rows

Moisture control:

• moisture control is necessary when placing the soil,

even though it may leave the borrow pit at the proper

water content

• The soil can be dried by spreading it in layers and

turning and aerating it in the sunshine36

Contd..

Fill Placement



• The pulverizing and mixing machines can help the

drying because they beat air through the soil during

the process

• Moisture addition is done by sprinkling water

followed by mixing with ploughs or the pulverizing

mixers

• If possible, moisture addition should be done several

hours before compaction, so that the soil will have

time for absorbing water 37

Contd..

Soil Compaction



• Compaction is the process by which the required

density is obtained in an embankment

• Two different phenomena are involved in this process:

1.Movement of the particles into new positions

2.Distortion of the particles, particularly the flaky ones

with their adsorbed layers, so that the voids between

them become smaller 38

Soil Compaction



Compaction methods:

• The oldest method of soil compaction is tamping. This

provides pressure at the moment of impact and some

vibration. Because of this dual action, tamping can be

used on all types of soil, but best in clayey sands,

sandy clays and similar soils of low (but notnegligible) cohesion

• The hard tamper , a block of iron, stone, or woodweighing 6 to 10 lb., is the simplest. The compaction

effort is small and so the soil must be compacted in

thin layers, from 1 to 2 in. 39

Contd..

Soil Compaction



• Pneumatic tampers , with the tamping part weighing

about 6lb., are faster than the hand tampers but are

only slightly more effective in producing high

compaction

• Jumping tampers weighing from 160 to 500lb.,

powered by gasoline, are more effective. These jumpfrom 6 to 18 in. in to the air and then drop back by

gravity to produce substantial impact pressure

• They are capable of compacting soils in layers of 6 to

12 in. thickness to densities of 100% of standard

proctor maximum at optimum moisture 40

Contd..

Soil Compaction



• The sheep foot roller consists of a steel drum with

projecting lugs or feet. It applies high static pressures

over a relatively small area: 1 to 9 sq. in. or an

equivalent diameter of 3 to 3.5 in.

• Rollers with foot pressures as low as 150 psi and as

high as 1,200 psi are available

• Because of high pressure and the small width of

application the sheep foot roller is the best adapted to

the highly cohesive soils like clays 41

Contd..

Soil Compaction



• Low plasticity soils can be compacted with the sheep

foot rollers but lower pressures must be employed to

avoid a bearing capacity failure

• A modified sheep foot roller with wider feet, 6 to 8 in.

across, is far more effective in silty soils

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Contd..

Soil Compaction



• The heavy pneumatic-tired roller consists of four large

pneumatic tires, mounted side by side on a

suspension system which permits them to move up

and down independently

• These support a ballast box which exerts loads of 50

to 200 tons depending on the types of tires, the box

sizes and the earth or rock ballast

• The pneumatic tired roller develops pressures of 80 to

150 psi over an area of 250 to 700 sq.in. or equivalent

diameters of 18 to 30 in 43

Contd..

Soil Compaction



• It is very well adapted to cohesive soils of low

plasticity, such as clayey sands and silts, and to

cohesion less soils such as sands and gravels where

the bearing capacity depends on width of the area of

application

44

Contd..

Soil Compaction



• Various forms of vibrating rollers are available. One is

a two-wheeled rubber-tired roller with a gasoline-

powered vibrator on the axis

• Another is smooth steel drum , ballasted with water,

and with a vibrating unit in the supporting frame

• Both the types are effective in compacting cohesion

less sands and gravels

• Layers from 12 to 18 in, thick can be compacted to

100% of the standard proctor maximum, often in one or

two passes of the machine 45

Contd..

Soil Compaction



46

Contd..

• Ordinary pavement rollers with steel drum wheels can

be used for soil compaction. They exert a moderate

degree of pressure across a wide drum

• They can be used in most soils provided the layersare very thin. Their main disadvantages are

¾ The tendency of the drum to bridge over small soft

pockets

¾ The formation of a ver smooth surface

Soil Compaction



• Flooding of sands and puddling of clays are

sometimes mentioned as compaction methods. While

these are better than no compaction at all, the degree

of compaction produced seldom exceeds 85% of the

standard proctor maximum

47

Contd..

Soil Compaction



Fill operation:

• The selection of the proper methods of compaction,

the layer thickness, pressure of the equipment, and

the best moisture is the joint responsibility of the

designer and the constructor

• The most important factor is the pressure distribution

in the layer being compacted. Based on Boussinesq

equation, the pressure beneath the compaction

device decreases with depth48

A hi h li d ll d

Soil Compaction



• A high pressure applied over a small area decreases

rapidly with increasing depth (as in a sheep foot roller)Fig. 6

• The tendency is to produce better compaction on the

surface of the compacted layer than at the bottom, i

the layer is very thick

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Fig. 6

Soil Compaction



• The wide compaction device with a smaller surface

pressure produces a more uniform pressure

throughout the layer. This is the characteristic of

rubber-tired roller

• The average pressure in a layer being compacted can

be increased by increasing the surface pressure or by

decreasing the layer thickness

• For earth dam cores, where uniformity is needed, the

moisture should be kept as high as possible. For the

shell where strength is most important, the moistureshould be as low as possible

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Soil Compaction



• The surface pressure, however, is limited by the

soil bearing capacity. Once that is exceeded, the

soil compaction is less

• Before any part of the embankment is compacted a

test strip should be set up where the best

equipment, moisture, layer thickness, and number

of passes are determined by experiment

• A little money spent on this may well save much

time, money and argument later 51

Contd..

Stage compaction:

Soil Compaction



Stage compaction:

• The maximum pressure which can be exerted by the

compaction equipment is limited by the soil bearing

capacity. The uncompacted soil with its loose

structure has very low bearing

• If heavy equipment is used in an attempt to compact

this soil, a bearing capacity failure is likely. If,

however, the soil is first partially compacted with thelight equipment, the bearing strength will be

increased sufficiently to permit the use of heavy

equipment52

Soil Compaction



Stage compaction:

• Such stage compaction can be very effective in higly

cohesive soils which require high pressures for

compaction, and it is useful for all soils when very

high degree of compaction are required

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Contd..

Documents

Construction of Earth Dam